1. Field of the Invention
This invention relates generally to igniters for emitting an electrical discharge to ignite a fuel-air mixture, such as corona igniters and spark plugs, and methods of forming the same.
2. Related Art
Igniters of corona discharge ignition systems and conventional spark discharge ignition systems typically include a center electrode formed of an electrical conductive material surrounded by a ceramic insulator. The center electrode typically extends into a combustion chamber and emits an electrical discharge, such as corona discharge or spark discharge. In a corona ignition system, an alternating voltage and current is provided, reversing high and low potential electrodes in rapid succession to enhance formation of the corona discharge. The center electrode of the corona igniter is charged to a high radio frequency voltage potential creating a strong radio frequency electric field in the combustion chamber. The electric field causes a portion of a mixture of fuel and air in the combustion chamber to ionize and begin dielectric breakdown, facilitating combustion of the fuel-air mixture. The electric field is preferably controlled so that the fuel-air mixture maintains dielectric properties and the corona discharge occurs, also referred to as a non-thermal plasma. The ionized portion of the fuel-air mixture forms a flame front which then becomes self-sustaining and combusts the remaining portion of the fuel-air mixture. Preferably, the electric field is controlled so that the fuel-air mixture does not lose all dielectric properties, which would create a thermal plasma and an electric arc between the electrode and grounded cylinder walls, piston, or other portion of the igniter. An example of a corona discharge ignition system is disclosed in U.S. Pat. No. 6,883,507 to Freen.
Corona igniters and spark plugs are oftentimes assembled such that the clearance between the center electrode and the insulator results in air gaps. Air or another gas from a surrounding manufacturing environment, or from a combustion chamber during operation of the igniter, fills the air gaps. During operation, when energy is supplied to the center electrode, the air in the gaps becomes ionized, creating and electrical field that leads to significant energy losses.